(1) Field of the Invention
This invention relates to an olefin-vinyl alcohol-vinyl acetal copolymer having excellent resistance to hot water and excellent oxygen barrier property in combination and also to a process for the preparation thereof. Further, the invention relates to a packaging material composed of said olefin-vinyl alcohol-vinyl acetal copolymer. Still further, the invention relates to a laminate structure having excellent resistance to hot water, excellent oxygen barrier property and high interlaminar peel strength in combination and more particularly, the invention concerns a laminate structure including a layer consisting of a novel olefin-vinyl alcohol-vinyl acetal copolymer.
(2) Description of the Prior Art
Ethylene-vinyl alcohols are ordinarily prepared by saponifying an ethylene-vinyl acetate copolymer. Accordingly, they are called "saponified ethylene-vinyl acetate copolymers". These saponified copolymers are broadly used as packaging materials for foods and medicines, especially for formation of packaging vessels. Properties of such saponified ethylene-vinyl acetate copolymers are changed depending on the ethylene content, the degree of saponification, the intrinsic viscosity and other factors, but they are excellent in the oxygen gas barrier property, the flavor-retaining property, the oil resistance and the transparency irrespective of these factors. Accordingly, when a film or other shaped article composed of such saponified copolymer is used as a structural material of a packaging vessel or as an oxygen barrier layer of a laminate material for production of a multi-layer vessel, decomposition or discoloration of the content by oxygen can be prevented and also deterioration of the taste or flavor or dispersion of the flavor can be prevented. Still further, bleeding of an oil or the like in the content to the surface of the vessel is not caused. Accordingly, various advantages are attained by the use of such saponified copolymers.
However, saponified ethylene-vinyl acetate copolymers are poor in the resistance to polar solvents such as water, and when they are exposed to hot water or steam as in the case of hot water sterilization or retort sterilization (heating sterilization under pressure) of foods, the mechanical strength is drastically reduced and such undesirable phenomena as deformation and adhesion are caused in films or vessels comprising these saponified copolymers, resulting in occurrence of such defects as blanching, wrinkling, contraction and interlaminar peeling. These defects are also observed even when such saponified ethylene-vinyl acetate copolymer is used as an intermediate layer of a multi-layer laminate structure including at least three layers so as to prevent direct contact of the copolymer with hot water or steam. Recently, in order to retain the flavor, color and texture of a packaged food after sterilization, there has been developed a so-called high-temperature short-time sterilization process (HTST process) in which sterilization is conducted under heating for a short time at a temperature higher than 130.degree. C. Under such high-temperature retort sterilization conditions, even dissolution of saponified ethylene-vinyl acetate copolymers is observed. Even sterilization using hot water maintained at a temperature of 80.degree. C. or higher is difficult in case of a film or vessel composed of a saponified ethylene-vinyl acetate copolymer and even when the saponified copolymer is used as an intermediate layer of a multi-layer laminate structure, defects such as blanching and wrinkling are caused under such high temperature conditions as adopted in retort sterilization and therefore, such laminate structure cannot practically be applied to the use where sterilization is required.
Accordingly, at the present, the application field of saponified ethylene-vinyl acetate copolymer is limited in the range where sterilization is not required, though they have various merits and advantages as packaging materials.
Various proposals have heretofore been made to improve the resistance to hot water in saponified ethylene-vinyl acetate copolymers. Among these proposals, a method comprising laminating an olefinic resin on both the surfaces of a film of a saponified ethylene-vinyl acetate copolymer is most frequently adopted in the art. A laminate structure obtained according to this method has a resistance to water of a relatively low temperature, but it cannot resist the above-mentioned high-temperature sterilization using hot water of a high temperature or steam.
Also various proposals have heretofore been made to improve the hot water resistance by treating saponified ethylene-vinyl acetate copolymers per se. For example, there can be mentioned a method comprising subjecting a film of the saponified copolymer to a treatment with hot water (see Japanese Patent Publication No. 13600/69), a method comprising dipping a film of the saponified copolymer in an aqueous solution containing sulfuric acid or hydrochloric acid and then subjecting the film to a treatment with hot water (see Japanese Patent Publication No. 114/71), a method comprising treating a shaped article of the saponified copolymer with a solution of an organic titanate (see Japanese Patent Publication No. 17071/71) and a method comprising treating a shaped article of the saponified copolymer with a solution comprising an alcohol, an organic carboxylic acid and an organic titanium compound (see Japanese Patent Publication No. 17915/71). However, according to these treatment methods, there are obtained only products which can resist the hot water treatment conducted at 100.degree. C. at highest, and it is impossible to obtain products which can resist the hot water or steam treatment conducted at 120.degree. C. or a higher temperature.
It has been known from old that when a film of a polyvinyl alcohol resin having an excellent oxygen gas barrier property is subjected to an acetalizing treatment using formaldehyde or the like, the hot water resistance of the film can be improved. However, when a film of such polyvinyl alcohol resin is acetalized to such an extent that the film can resist the retort sterilization conducted at a temperature higher than 120.degree. C., the inherent oxygen gas barrier property of the film is drastically degraded and no excellent packaging material is obtained.